In my copending application Ser. No. 863,624 of even date there has been disclosed a low-pass or band-pass microwave filter with a multiplicity of cascaded sections or cells each having a parallel-resonant series arm. Such a cavity resonator is frequently designed as a hollow cylindrical body of conductive material with an inlet for coupling its cavity to an associated guide path and with a substantially closed opposite end, allowing microwaves to propagate in the cavity in the mode TE.sub.01 which minimizes the insertion loss. As is well known, the lines of current flow in the TE.sub.01 mode lie in planes perpendicular to the axis of a cylindrical cavity or waveguide; in an alternate mode TM.sub.11 these current lines lie in planes parallel to the axis.
Conventional cavity resonators of the aforedescribed type, when excited in the TE.sub.01 mode, also give rise to the parasitic TM.sub.11 mode. With the cavity resonating at the same frequency in both modes, the frequency-selective quality of the resonator is impaired. It is therefore customary to provide means in such a cavity for reducing the proportion of wave energy propagating in the TM.sub.11 mode in favor of the preferred TE.sub.01 mode. This is conventionally accomplished by inserting into the cavity cylinder a conductive piston of smaller diameter spaced from its wall, with interposition of energy-absorbing loss material between the piston and the closed end of the cavity. As the loops of the axially extending lines of current in the TM.sub.11 mode are closed through the loss material, that mode is weakened.
Such a construction involves certain technical problems and adds rather considerably to the cost of the filter while also imposing restrictions on the designer. Moreover, especially with high powers in the kilowatt range, even the small percentage of absorbed energy may cause an inadmissible increase in temperature.
Other conventional solutions to the stated problem involve the provision of radially extending screws or other conductive projections on the inner cavity wall for the purpose of distorting the magnetic field of the parasitic mode or modes without significantly affecting the desired principal mode. Such a cavity is generally tuned to a fixed resonance frequency.